Chemisorption properties

At this point, we can undertake the study of chemisorption on a supported metal. Despite the importance of this process to catalysis, quantum-mechanical studies have been somewhat scarce. The problem was first investigated by Ruckenstein and Huang (1973), who formulated a general MO 

A few years later, Davison et al (1988) applied the ANG model of chemisorption to supported-metal catalysts. The key parameters were found to be the metal film thickness and the metal-support bond strength. Related papers followed, studying impurity effects (Zhang and Wei (1991), Sun et al (1994b)) and variation with metal substrate (Xie et al (1992)). 

Parallel studies were performed by Liu and Davison (1988), who investigated the process of chemisorption on inverse-supported catalysts, where the surface him is a semiconductor and the underlying support is a metal. Again a key parameter was found to be him thickness, and the substrate was observed to behave as either an acceptor or a donor, depending upon that thickness. The lack of charge self-consistency in this work was addressed by Sun et al (1994a), who also studied the effects of thickness and different metal constituents. 

A different approach was taken by Hao and Cooper (1994), who used a combination of the him linear muffin-tin orbital (LMTO) method and an ab initio molecular quantum cluster method, to investigate S02 adsorption on a Cu monolayer supported by 7—AI2O3. Emphasis here was on the geometry of adsorption sites, with the conclusion that the preferred adsorption site is the Al—Al bridging one. 

The problem at hand is the application of the ANG model (Chap. 4) to the adatom-metal-support system, which is shown schematically in Fig. 5.2. The adatom (H) at site m = a, has electronic energy, eaa (4.34) and is connected by a bond of energy [3 to the surface atom (Ni), at site m = 0. The support is ZnO, with a Zn atom at the interface site m = n + 1 (Davison et al 1988). 

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To understand the chemisorptive property of lr/ln/H-ZSM-5 in a comparison with that of ln/H-ZSM-5, the chemisorption data were analyzed in detail. The data shown in Fig. 3 are relatively well fitted to the Langmuir isotherm, as shown in Fig. 5. From these relations, the equilibrium constant (K) and the amount of NO2 adsorbed at saturation (Vo) were determined according to the following equations ... 

Pacchioni G, Illas F. 2003. Electronic structure and chemisorption properties of supported metal clusters model calculations. In Wieckowski A, Savinova ER, Vayenas CG. editors. Catalysis and Electrocatalysis at Nanoparticle Surfaces. New York Marcel Dekker. 

Narita, T., Miura, H., Ohira, M., Hondou, H., Sugiyama, K., and Matsuda, T. 1987. The effect of reduction temperature on the chemisorptive properties of Ru/Al203 Effect of chlorine. Appl. Catal. 32 185-90. 

Among the various types of composite systems, that of the metal-support ranks as one of the most important, because of its crucial role in catalysis. The situation under consideration is that of chemisorption on a thin metal him (the catalyst), which sits on the surface of a semiconductor (the support). The fundamental question concerns the thickness of the film needed to accurately mimic the chemisorption properties of the bulk metal, because metallization of inexpensive semiconductor materials provides a means of fabricating catalysts economically, even from such precious metals as Pt, Au and Ag. 

In conclusion, metal-support substrates provide good examples of composite systems that can be studied efficiently by GF techniques. The key parameter is clearly seen to be the film thickness it controls the extent to which the metal-support system mimics the chemisorption properties of the pure metal. Also important is the bond strength (7) between the metal and the support, as it governs the flow of charge between the metal and the support, thus determining the amount of charge available at the surface to partake in chemisorption. 

Turning to the numerical results (Sulston et al 1986), we look at the chemisorption properties of H on Cu/Ni and Au/Pt alloys, over a range of bulk concentrations. The H parameters used are ea = —14.3 eV, measured from the vacuum level, and U = 12.9 eV. The pure-metal parameters (Newns 1969 and Nordlander et al 1984) are shown in Table 6.1. Following the concept of the VCA, f and 7 are assigned concentration dependencies, namely,... 

However, surface segregation (cs / Cb) has a radical effect on AE, as can clearly be seen in Fig. 6.3(b). Cu/Ni alloys are known (Kelley and Ponec 1981, Ouannasser et al 1997) to have an enriched Cu concentration in the surface layer for all bulk concentrations. As a result, the alloy shows a more Cu-like behaviour than it would if it were non-segregated. In particular, AE has a value significantly closer to that for pure Cu than in the case where cs = Cb, and this occurs at all bulk concentrations c. The smallest change in AE occurs in Cu-rich alloys, which is understandable, because these alloys have mostly Cu in the surface layer anyway, so the effect of surface segregation is relatively small. Thus, surface segregation has a lesser effect in these alloys than in Ni-rich ones, which have mostly Ni in the bulk, but may have a Cu majority in the surface layer. Clearly, then, the concentration cs of the surface layer is the primary parameter in determining the chemisorption properties of the DBA. 

In conclusion, we have seen that alloys can exhibit a variety of interesting chemisorption properties. The chief parameters determining the behaviour of a system are the concentrations of the various layers, especially the surface one. Other important parameters are the effective electronic energy, the occupied band width, the adatom bond strength and the adatom position. 

As we have seen, the chemisorption properties of the substrate depend on its electronic structure, so that changes in the latter are reflected in the former. In the case of electrified substrates, the strength of the applied electric field governs the substrate modification and, thereby, regulates the chemisorption process in a controllable manner. 

Summarizing, it is clear that the indirect interaction between adatoms has a significant effect on the chemisorption properties of the system. Most noticeably, the chemisorption energy has a damped, oscillatory dependence on the adatom separation, as first quantified in (8.1) by Grimley. Thus, multi-atom adsorption occurs preferentially with the atoms at certain sites relative to one another. 

There is a one-point modification of a chemisorption method, which is widely used for measurements of Ac. In this case, only one adsorption point of a chemisorption isotherm is measured, and is compared with only one point on a chemisorption isotherm on a reference material (usually, powder [black] or foil). The identity of the chemisorption properties of the active components in supported and pure form is postulated, but very often does not fulfill, making one-point modification an inaccurate procedure, which can hardly be used in scientific studies. For example, studies of supported Rh catalysts by 02 and CO chemosorption have shown that three different blacks of Rh yield three different results [88], The multipoint comparison of chemisorption isotherms shown that only one black had a chemisorption isotherm that had affinity to the isotherm on a supported metal. 

C. T. Campbell, Ultrathin metal films and particles on oxide surfaces Structural, electronic and chemisorptive properties, Surf. Sci. Rep. 27(1-3), 1-111 (1997). 

H. Topsbe, N. Topsbe, H. Bohlbro, and J. A. Dumesic, Supported iron catalysts Particle size dependence of catalytic and chemisorptive properties, Proc. 7th Int. Congress Catalysis, edited by T. Seyama, K. Tanabe (Kondansha, Tokyo), p. 247 (1981). 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

Table 6.3 Mean particle size d) of metallic and bimetallic particles measured by TEM and H2 and CO chemisorption properties of selected catalysts. (Reproduced from Reference 147].)...

Table 6.5 H2 and CO chemisorption properties and XPS results for PtSn-BM, PtSn-OM and PtSn-OM systems.(Reproduced from Reference [30].)...

Dynamic smdies of the alloy system in CO and H2 demonstrate that the morphology and chemical surfaces differ in the different gases and they influence chemisorption properties. Subnanometre layers of Pd observed in CO and in the synthesis gas have been confirmed by EDX analyses. The surfaces are primarily Pd-rich (100) surfaces generated during the syngas reaction and may be active structures in the methanol synthesis. Diffuse scattering is observed in perfect B2 catalyst particles. This is attributed to directional lattice vibrations, with the diffuse streaks resulting primarily from the intersections of 111 reciprocal lattice (rel) walls and (110) rel rods with the Ewald sphere. 

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